The interconnection of electrical cables or the termination of an electrical cable requires removal of the cable shield along some portion of the cable length. Removal of the shield induces electrical stresses along the cable axis.
Such stresses are typically reduced by applying a field grading material to the interconnection or termination. Capacitive field grading materials are employed in alternating current (AC) applications, while resistive field grading materials may be employed in AC or direct current (DC) applications.
When a resistive field grading material is applied to an unshielded portion of the cable and adjacent the remaining shield, a positive voltage applied to the cable induces a resistive voltage drop in the field grading material, which distributes the potential more uniformly. This distribution is more uniform if the resistance or conductivity of the field grading material itself is non-linear.
Known field grading materials typically comprise semiconducting ceramic particles, such as silicon carbide (SiC) or zinc oxide (ZnO) and carbon black admixed in a polymer material. Achieving non-linear conductivity in such materials typically requires high loading (e.g., 30% to 40% by volume) of these particles within the polymer. As a result, however, the field grading materials tend to suffer from poor mechanical properties (e.g., brittleness), have a relatively high weight, and a tendency to overheat at high electrical fields.